The unrelenting rise in coal use without deployment of carbon capture and storage (CCS) is fundamentally incompatible with climate change objectives. The world faces an unabated global demand for energy, both for livelihood and for pure economic growth, as well as an existing, sizeable, carbon-intense infrastructure. There is no rational near-term energy future that does not involve continued use of fossil fuel. Maintaining coal-fired power generation would make practical sense if control of carbon dioxide (CO2) could be made affordable. Current CCS technologies will not only increase capital costs but also impose significant performance penalties, challenging the competitiveness of coal power generation. Furthermore, many locations worldwide lack suitable geology for CO2 storage, one of several factors expected to constrain CCS deployment.
Applicant attempted to solve this predicament as disclosed in patent applications entitled “Sulphur-assisted Carbon Capture and Storage (CCS) Processes and Systems” published as WO2014/117243, and also published as EP 2 950 911; CA 2,898,519; and US 2015/362188, and referred herein as a Hybrid Energy System or HES, in which CO2 is recirculated by way of conversion to an intermediate sulphur compound, that is carbonyl sulphide (COS). The conversion enables utilization of the enormous latent chemical combustion energy value of sulphur (S) to generate complementary electric energy for the various energy-consuming steps in the CCS processes.
Carbonyl sulphide when reacted with sulphur dioxide (SO2) reduces it back to sulphur and CO2. Thus, the sulphur as a fuel feedstock is recycled, producing neither any detrimental environmental impact of sulphur oxides nor any solid waste. Application of this concept for power generation is virtually universal and a wide variety of arrangements or modifications to the proposed system are possible.
Sulphur Assisted CCS Capabilities and Shortcomings
The recirculation of carbon, which constitutes an underlying concept of carbon capture and utilization (CCU), aims to improve the economic viability of carbon capture. This may well result in accelerating intermediary measures to drive CCS deployment. However, CCU can contribute to alleviating global CO2 emissions only if the recirculated CO2 has come from power plants or industry—not from natural geologic sources as is common with, for example, conventional enhanced oil recovery (EOR). In addition, the energy required by the carbon capture and utilization process should come from carbon neutral sources.
In the cited disclosure, the specific prerequisite for the utilization of sulphur as a fuel is that the CO2 already captured from power plants and/or industrial facilities be converted to an intermediate sulphur compound by an industrially proven process of catalytic oxygen/sulphur (O/S) exchange reaction with a common industrial solvent carbon disulphide (CS2). However, the necessity of adding the CO2 conversion system to the post-combustion CO2 capture system increases complexity and raises the capital and operating costs of the power plant.
Moreover, to convert substantial quantities of carbon dioxide cost-effectively requires massive scale CS2 manufacture. Fortunately, carbon disulfide can be synthesized from plentiful waste materials that are found around chemical, petroleum and other industries. As an example, in the Canadian oil sands, carbon disulphide can be rapidly and satisfactorily produced by the utilization of massive generated waste such as petroleum coke and large stockpiles of sulphur by using methods disclosed in various patent literature. Also, it can be alternatively formed by utilization of H2S from a gas stream containing lower molecular weight alkanes derived in the processing of tar sands (see, e.g., patent applications CA 2,864,792, and US 2013217938). However, the drawback of employing the carbon disulphide for the CO2 conversion is that the carbon from CS2 substantially increases the quantities of CO2 equal to the volume being converted.
To avoid increasing the volume of CO2, the conversion can be performed by the catalytic reaction of CO2 with hydrogen sulphide (H2S). This reaction is known in the art, and may be carried out in any suitable way known in the art. Typically, the reaction will be carried out by contacting gaseous carbon dioxide and gaseous hydrogen sulphide with a catalyst, in the presence of a sorbent. This specific method of CO2 conversion to COS was commercially employed by Shell (see U.S. Pat. No. 4,671,946) at the North Sea Gas Terminal Emden, Germany as a conditioning method prior to distribution of the natural gas contaminated with a lean volume of H2S. However, a more efficient catalyst system must be developed before this method can be applied for the purpose of CO2 conversion.
Even so, with the substantial increase in quantities of CO2 the HES can contribute to alleviating global CO2 emissions and create value as depicted in FIG. 1, when the particular volume of the CO2 captured from the power plant is converted to methanol, and the CO2 from CS2 is sequestrated. The supplemental electric energy can provide power for the electrolysis of water to produce the required hydrogen and to pressurize the CO2 for transportation and storage. The ability to utilize the oxygen from the electrolysis for sulphur combustion can significantly improve the economic viability of HES. It is important to note here that electrolysis is one of the most efficient ways to get hydrogen from any form of water, whereby electricity can be converted into hydrogen with more than 80% efficiency.
Moreover, the methanation reaction is exothermic, and therefore a surplus of heat is generated in the process which can be utilized for the carbon capture process. In the case of carbon capture by chemical absorption with amine-based solution, the main energy demand arises from the regeneration of the rich solutions, which is achieved by the heating of the scrubbing liquids. It can be also used for the heating of water for the electrolysis, which will decrease the electricity requirement.
Another example of HES application is to provide energy for cryogenic oxygen generation. This implementation offers the possibility of rapidly retrofitting existing coal power plants to oxyfuel systems with the lowest costs compared to other zero emission technologies. The urgency of CCS retrofitting is further exacerbated by the significant lifetime of existing power plants and the very large number of plants likely to be built over the coming decades worldwide without CO2 emissions abatement.
Then again, the preferred techniques for capturing CO2 in cement plants are oxyfuel and post-combustion capture. However, CO2 capture by oxyfuel technology will increase the cement production cost by around 40% (excluding CO2 transport and storage costs) and post combustion liquid solvent scrubbing will increase the cost by around 70-100% (Annual Review 2013, www.ieaghg.org.). The same review concluded that post-combustion CO2 capture (i.e. capture of CO2 from different flue gases of the different combustion processes) in an integrated steel mill could be cost prohibitive for the reasons that it significantly increases the energy demand of the steel mill. The leading use of Oxy-Blast Furnace (OBF) Technology is one of the technology options considered to provide a significant reduction of CO2 emissions from iron and steel production based on a blast furnace (BF) and basic oxygen furnace (BOF) route. In both of the above presented cases, the implementation of the HES as an energy provider can be the key for viable CCS for these industries.
Furthermore, one of the most significant enhancements of CCS by CO2 conversion is that the COS compound introduces a flexibility that permits a much simpler, more energy-efficient means of CO2 transportation when compared to a method in a supercritical phase. For example, the intermediate COS at 10° C. and 9 bar is a liquid with a density of 1 gm/cc and contains 0.2 gm carbon per cc, while at the same temperature and pressure CO2 would be a vapour with a density of 0.018 gm/cc or 0.005 gm carbon per cc.
Thus, as depicted in FIG. 1, the CO2 conversion plant or/and sulphur-fueled power plant can be foreseen as a hub for a CCS cluster of carbon-emitting facilities (e.g. steel, cement, lime, chemical industry, refining, and coal power plants) while simultaneously utilizing HES for addressing the hub participants' various energy-consuming steps in the CCS process. The ability to share a transport and storage network infrastructure is a major component of CCS cost reduction.
Moreover, the HES could be located either onshore and/or on vessels conveying the CO2 or COS to offshore storage as depicted in FIG. 2. Ships offer flexibility in the CO2 chain unlike pipelines. Transport by ship can provide flexibility in combining CO2 from several sources, in changing capture sites, storage sites and the transportation routes in a CCS project, an attractive and viable alternative to overcome the limitations imposed by a “sink-source matching condition.”
While pipelines require large capital expenditures up front, this is not the case with ships. Ships, on the other hand, have higher operating costs. The largest shipping cost components are electricity and fuel, each accounting for almost 30% of the total cost. Capital costs only contribute around 28% of the total shipping cost, compared to more than 70% for pipeline transport. By employing the HES for powering ship engines (steam/gas turbine), the logistics of transporting CO2 to offshore storage areas will become economically feasible.
Hydrogen and Renewable Energy Sources Issues
The largest potential for the utilization of considerable quantities of CO2 is in the process of making hydrocarbons that requires a supply on a massive scale of hydrogen. How to obtain the hydrogen still remains as an enduring challenge. The main obstacle for abundant production of H2 by electrolysis is the high cost of electricity compared to petrochemical methods such as steam reforming of methane (natural gas), a source that is cheap but hardly green. The high cost of hydrogen production using electrolysis led to the search for a less expensive technology, one of which is the thermochemical cycle.
Thermochemical cycles are processes in which water is decomposed into hydrogen and oxygen via chemical reactions using intermediate elements which are recycled. The leading thermochemical processes that all have common high temperature reaction of thermal decomposition of sulphuric acid are three sulphur water-splitting cycles: Sulfur-Iodine (SI) process, Hybrid Sulfur (HyS) or Westinghouse process, and Ca—Br process (ANL modification of UT-3 cycle). The water-splitting cycles consist of a series of linked chemical reactions which result in the dissociation of water molecules into hydrogen and oxygen. All of the intermediate chemicals are regenerated and the only consumable is water.
Among these, the two-step Hybrid Sulfur (HyS) cycle presented schematically in FIG. 4 is one of the simplest, all-fluid thermochemical cycles that have been demonstrated at a laboratory scale to confirm performance characteristics. It was patented by Brecher and Wuin U.S. Pat. No. 3,888,750 in 1975 and extensively developed by Westinghouse in the late 1970s and early 1980s.
The key component of the HyS process is the electrolyser, also called a SO2-depolarized electrolyser (SDE) where hydrogen (H2) and sulphuric acid (H2SO4) are produced as products of the reaction between water and dissolved SO2:SO2+2H2O→H2SO4+H2, electrolysis (25-100° C.)  (1)
The sulphuric acid is then decomposed at high temperature into sulphur dioxide, oxygen (O2) and water (H2O):H2SO4→SO2+½O2+H2O thermochemical (800-1000° C.)  (2)
The presence of sulphur dioxide along with water in the electrolysis reduces the required electrode potential to well below that required for electrolysis of pure-water, thus reducing the total energy consumed by the electrolysis. In practice, SO2 electrolysis may require no more than 25% of the electricity needed in the alkaline water electrolysis, although at the expense of the need to decompose H2SO4 at high temperatures in order to recycle the SO2.
The decomposition of SO3 to SO2 is thermodynamically unfavourable at lower temperatures, so it is carried out at temperatures above 800° C. in order to produce a sensible equilibrium conversion. To be feasible, the process was designed to be coupled with a very high temperature nuclear power plant, which would supply both the heat needed for the sulfuric acid concentration and decomposition steps and the electricity required for the electrochemical part. A very high temperature reactor belongs to the group of “Generation IV” nuclear power plants, which have yet to be constructed. Additionally, the cycle needs an expensive chemical plant.
The coupling of the HyS cycle with a solar heat source has also been studied in an attempt to achieve sufficiently high temperatures for sulfuric acid decomposition. Water electrolysis powered by renewable energy resources would produce only hydrogen and oxygen, avoiding the emission of CO2; however, renewable energy resources alone are inadequate.
Producing electricity from direct solar radiation or wind is limited by the unpredictability and variability of these sources. Currently, wind power is the fastest growing renewable energy source, especially in Europe. For example, in Denmark, over 20% of the demand for electricity is generated by wind power. However, at an optimum location, generally offshore, a windmill-driven generator will only run at its nominal power during 30% of the time, while at most land-based locations wind generators typically operate at nominal power 20% of the time.
Compensating for the rapid fluctuations in output of large-scale wind and solar-based generators is difficult for the conventional steam-based power plants, lowers the utilisation factor of the other power plants, which increase the capital costs per kWh. Running conventional fuel-based power plants at a low load drastically increases their fuel consumption, increases their CO2 emission, and drastically increases their maintenance costs per kWh.
Technologies which provide these capabilities are in place, e.g., gas engines, which are low in emissions, quick and flexible and also allow heat recovery and energy storage integration. Yet, with coal still being the cheapest fuel in most parts of the world, natural gas has a hard time to compete.
Furthermore, the CCS concept assumes that the station is running at a constant level of power generation and carbon emissions. As such, there is yet no method to alleviate the effect of changes in demand on the CCS.
Switching from fossil fuels to bioenergy does not necessarily reduce CO2 emissions overall. Depending on how the biomass is produced and used, the resulting emissions and climate impact can be better or worse when compared to fossil fuels. The JRC, the European Commission's in-house science service, states that “the assumption of biogenic carbon neutrality is not valid under policy relevant time horizons” (Carbon accounting of forest bioenergy, 2013). In addition, the US Environmental Protection Agency recognises that “carbon neutrality cannot be assumed for all biomass energy a priori (Framework for Assessing Biogenic CO2 Emissions from Stationary Sources, 2014).
The main environmental drawback of large-scale electricity generation from geothermal energy (specifically in volcanic areas such as Iceland) is that the wells contain high amounts of COz2, derived from metamorphism of carbonate, which produce worldwide average emissions of 122 g CO2 per generated kWhe.
For a short- to medium-term application, a new alternative Outotec® open cycle process (OOC) has been proposed for hydrogen production. This process involves only one stage (SDE) and does not require sulfuric acid decomposition. The SO2 used in the process can be obtained from flash smelting, sulfides roasting, sulfur combustion or any other similar operation, and because sulfuric acid is a commercial product, the cycle may be left open.
Although various systems and methods for carbon neutral energy and hydrogen production are disclosed in the prior art, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide methods and systems that provide an improvement over the prior art.